1. FIELD OF THE INVENTION
The invention concerns a propeller in particular for a ship and having blades each of which is pivotally arranged in the hub of the propeller so that the blade is capable of pivoting to and fro in an axial plane between forward and rearward positions.
2. DESCRIPTION OF THE PRIOR ART
The energy which a propeller is capable of releasing when it is to propel a ship forwardly in the water depends upon the configuration of the propeller, in particular diameter, area, pitch and number of blades, in addition to the relative velocity of flow of the water and the actual number of revolutions of the propeller. The ratio of the energy released to the energy received by the propeller from the drive engine is called the efficiency of the propeller which is thus an expression of the ability of the propeller to utilize the added energy. When the ship is propelled by power the fuel consumption should be restricted to what is strictly necessary for economic and environmental reasons. Therefore, it will usually be attempted to design a propeller for a specific purpose such that it has an efficiency as high as possible under the given conditions of operation. One of the most important parameters included in the calculations in this context is the shape of the blade profiles formed as intersecting faces between the blades and cylinder faces which extend coaxially with the propeller. It is known that the intended good efficiency can be obtained by providing these blade profiles with a curved central line between their edges and having the convex side of this line facing the same way as the direction of navigation.
The typical direction of navigation will usually be forward, which is therefore generally chosen as the way which the convex side of the profile central lines is to face. Forward navigation can therefore take place with a good efficiency, which, on the other hand, is immediately transformed to an extremely poor efficiency when the ship is to be propelled rearwardly, because the curvature of the profile then faces the wrong way. In propellers having rigid blades the curvature and its direction are given once and for all when the propeller was manufactured, while the curvatures in propellers having pivotable blades are changed in response to the position of these.
U.S. Patent specification 3,981,613 discloses a folding propeller which is constructed particularly for improving the efficiency during rearward navigation. However, this is achieved merely by permitting the blades to pivot to positions which are located on both sides of a plane which contains the axis of rotation of the blades and is at right angles to the propeller axis.
It is common to the above-mentioned conventional propellers that their average efficiency in forward as well as rearward navigation is not satisfactory, and that the disadvantageous shape of the blades during rearward navigation is moreover the cause of strong propeller noise and vibrations which spread into the ship to the inconvenience of those on board.
Accordingly, there is a pronounced need for a new propeller structure whose total efficiency is optimum in forward and rearward navigation, and which, also in the latter situation, has blade profiles imparting a quiet and steady operation to the propeller.